1. Field of the Invention
The present invention relates to a semiconductor device such as an integrated gate bipolar transistor (IGBT) and an integrated circuit (IC) with a built-in IGBT.
2. Description of the Background Art
Generally, the equivalent circuit of a collector-shorted IGBT is structured such that the base and emitter of a PNP transistor, between which the drain and source of an N-channel MOSFET are connected, are short-circuited via a resistance (first conventional example).
In order to turn on the IGBT of this kind, when the collector of the IGBT (the emitter of the PNP transistor) is at a higher potential than the emitter of the IGBT (the source of the N-channel MOSFET), a predetermined positive voltage is applied to the gate of the IGBT (the gate of the N-channel MOSFET). Thereby, the N-channel MOSFET is turned on, whereby electrons are injected from the emitter of the IGBT through the N-channel MOSFET into the base of the PNP transistor, and holes are injected from the collector of the IGBT through the emitter of the PNP transistor into the base of the PNP transistor. This electron and hole injection causes conductivity modulation of the PNP transistor and reduces the turn-on voltage of the PNP transistor, thereby turning on the PNP transistor.
On the other hand, in order to turn off the IGBT of this kind, the application of a predetermined positive voltage to the gate of the IGBT is terminated. This stops the electron and hole injection into the PNP transistor, thereby decreasing electron and hole densities in the PNP transistor and increasing the turn-on voltage of the PNP transistor. Accordingly, the PNP transistor is turned off.
When the emitter of the IGBT is at a higher potential than the collector of the IGBT, the IGBT of this kind conducts current from its emitter to collector through a parasitic diode in the N-channel MOSFET and through the resistance between the base and emitter of the PNP transistor (reverse conducting capability). This reverse conducting capability is essential when the IGBT is adopted as an inductance load.
In the case where an IGBT with no reverse conducting capability (i.e., non-collector-shorted IGBT) is adopted as an inductance load, an external diode needs to be connected in reverse parallel between the collector and emitter of the IGBT (second conventional example).
Such conventional examples can be found in conventional art, for example in Japanese Patent Application Laid-open No. 9-82961 (1997).
The first conventional example given above has the disadvantage that a higher value of the resistance between the base and emitter of the PNP transistor results in a higher conducting resistance during reverse conduction and thus inhibits the reverse conducting capability.
On the other hand, a lower value of the resistance, during turn-on of the IGBT, disadvantageously causes both electrons from the emitter side of the IGBT and holes from the collector side of the IGBT to flow into the resistance without flowing into the base of the PNP transistor. This makes the electron and hole injection into the PNP transistor difficult and slows down the drop in the turn-on voltage of the PNP transistor, thereby delaying the turning on of the IGBT.
On the contrary, a lower value of the resistance, during turn-off of the IGBT, advantageously causes electrons and holes, which are accumulated in the base of the PNP transistor, to be emitted more quickly from the base of the PNP transistor through the resistance. This results in a rapid drop in the turn-on voltage of the PNP transistor, thereby speeding up the turning off of the IGBT.
The second conventional example given above has a disadvantage of higher cost because it requires an external diode whose breakdown voltage and operating current need to be equivalent to those of the IGBT and thus which is about the same size as the IGBT.